A pair of conductive lines are coupled when they are spaced apart, but spaced closely enough together for energy flowing in one to be induced in the other. The amount of energy flowing between the lines is related to the dielectric medium the conductors are in and the spacing between the lines. Even though electromagnetic fields surrounding the lines are theoretically infinite, lines are often referred to as being closely or tightly coupled, loosely coupled, or uncoupled, based on the relative amount of coupling.
Couplers are electromagnetic devices formed to take advantage of coupled lines, and may have four ports, such as one port associated with each end of two coupled lines. A main line has an input connected directly or indirectly to an input port. The other end is connected to the direct port. The other or auxiliary line extends between a coupled port and an isolated port. A coupler may be reversed, in which case the isolated port becomes the input port and the input port becomes the isolated port. Similarly, the coupled port and direct port have reversed designations.
Directional couplers are four-port networks that may be simultaneously impedance matched at all ports. Power may flow from one or the other input port to a corresponding output port or output ports, and if the output ports are properly terminated, the ports of the input pair are isolated. A hybrid coupler may generally be assumed to divide the output power equally between the outputs, whereas a directional coupler, as a more general term, may have unequal outputs. Often, the coupler has very weak coupling to a coupled output, which reduces the insertion loss from the input to the main or direct output. One measure of the quality of a directional coupler is its directivity, which is a measure of the desired coupled output to an isolated port output.
Adjacent parallel transmission lines can couple both electrically and magnetically. The coupling is inherently proportional to frequency, and the directivity can be high if the magnetic and electric couplings are equal. Longer coupling regions can increase the coupling between lines, until the vector sum of the incremental couplings no longer increases, and the coupling will decrease with increasing electrical length in a sinusoidal fashion. In many applications it is desired to have a constant coupling over a wide band. Symmetrical couplers exhibit inherently a 90-degree phase difference between the coupled output ports, whereas asymmetrical couplers have phase differences that approach zero-degrees or 180-degrees.
Unless ferrite or other high permeability materials are used, greater than octave bandwidths at higher frequencies are generally achieved with cascading couplers. In a uniform long coupler the coupling rolls off when the length exceeds one-quarter wavelength, and only an octave bandwidth is practical for +/−0.3 dB coupling ripple. If three equal length couplers are connected as one long coupler, with the two outer sections being equal in coupling and much weaker than the center coupling, a wideband design results. At low frequencies all three couplings add. At higher frequencies the three sections can combine to give reduced coupling at the center frequency, where each coupler is one-quarter wavelength. This design may be extended to many sections to obtain a very large bandwidth.
Two characteristics exist with the cascaded coupler approach. One is that the coupler becomes very long and lossy, since its combined length is about one-quarter wavelength long at the lowest band edge. Further, the coupling of the center section gets very tight, especially for 3 dB multi-octave couplers. A cascaded coupler of X:1 bandwidth is about X quarter wavelengths long at the high end of its range. As an alternative, the use of lumped, but generally higher loss, elements has been proposed.
An asymmetrical coupler with a continuously increasing coupling that abruptly terminates at the end of the coupled region will behave differently from a symmetrical coupler. Instead of a constant 90-degree phase difference between the output ports, close to zero or 180 degrees phase difference can be realized. If only the magnitude of the coupling is important, this coupler can be shorter than a symmetric coupler for a given bandwidth, perhaps two-thirds or three-fourths the length.
Most cascaded-line couplers, other than lumped element versions, are designed using an analogy between stepped impedance couplers and transformers. As a result, the couplers are made in stepped sections that each have a length of one-fourth wavelength of a center design frequency, and may be several sections long. The coupler sections may be combined into a smoothly varying coupler. This design theoretically raises the high frequency cutoff, but it does not reduce the length of the coupler.